The present invention relates generally to a method of geophysical exploration including processing and displaying seismic data to obtain a measure of subterranean formation rock properties. Seismic data including a plurality of seismic signals or traces are obtained with sets of seismic sources and seismic receivers. A set of observed attributes, quantitatively descriptive of variations in the seismic signal amplitude as a function of incident angle, are obtained for selected seismic events. The observed set of attributes provides a measure of the contrast in formation rock properties across subterranean formation interfaces associated with each selected seismic event. The set of observed attributes can be transformed to provide a most probable estimate of the subterranean formation rock properties. Additionally, a diagnotic technique is provided for interpreting relative formation lithology and pore fluid content.
In the continuing search for hydrocarbons contained in the earth's subterranean formations, exploration geophysicists have developed numerous techniques for imparting seismic wave energy into the earth's subterranean formations; recording the returning reflected seismic wave energy and processing the recorded seismic wave energy to produce seismic signals or traces. Such seismic signals or traces contain a multiplicity of information, e.g., frequency, amplitude, phase, etc., which can be related to formation structure, lithology, or pore fluid content. More recently, geophysicists' interest have turned to evaluating high intensity seismic amplitude events in the seismic signals or traces, i.e., "bright spots" and variations in the seismic signal amplitude as a function of range for selected seismic events. Exemplary of such focus are Quay, et. al., U.S. Pat. No. 3,899,768; Thompson, et. al., U.S. Pat. No. 4,375,090, and Ostrander, U.S. Pat. Nos. 4,316,267 and 4,316,268.
In particular, Ostrander indicates that progressive changes in the seismic signal amplitude of a high intensity seismic event, as a function of range, is more likely than not an indicator of a gas-bearing formation. Specifically, progressive seismic signal amplitude changes, in an increasing or decreasing manner, with increasing range is the criteria for identifying gas-bearing formations. Ostrander discloses a method for signal enhancement to improve the visual resolution of such progressive changes in seismic signal amplitude as a function of range.
Quay recognizes that lateral variations in the seismic data can be attributed to variations of the lithological character of the subterranean formations. Quay obtained such results by extracting selected seismic parameters from a seismic wave and thereafter displaying such seismic parameters upon a seismic trace of such seismic data. The visual correlation of events in such seismic parameters relative to the structural interpretation of the seismic trace yielded a scheme for interpreting seismic record sections.
Thompson discloses that acoustic characteristics associated with hydrocarbon-containing formations can be compared with similar synthetic values.
Although evaluation of bright spots has been used to indicate gas reservoirs throughout the world, such analysis is still a calculated risk, as evidenced by the significant number of such events which are nonproductive when actually drilled.